1. Field
This disclosure relates to compression of electronic image signals, more particularly to removal of distortions in reconstructing of these compressed image signals.
2. Background
Block-based DCT (Discrete Cosine Transform) coding has been used in many image compression standards, such as H.261, H.263, JPEG, MPEG-1, and MPEG-2. This is a lossy compression technique, meaning that there are variations in the output image when compared to the original image. The compression process causes visual artifacts in the decoded images, two of which are referred to as blocking and ringing. Blocking artifacts occur along the block boundaries and ringing artifacts appear near strong image edges, both due to the coarse quantization of the DCT coefficients.
Among the coding artifacts, ringing is the most visible one in high quality systems, those having low compression ratios. One typical example of the ringing artifacts is the noise surrounding strong image edges that represent text and graphical object boundaries. In video coding, due to slight variations from frame to frame, ringing noise is visible in moving pictures as a local flickering near edges. This type of noise is known as “mosquito noise.”
In general, for a given compression ratio, there is a limitation on the visual quality of an image. Quantization tables can be carefully designed to get higher quality images, however, the tradeoff is to get lower compression ratio. Therefore, post-processing techniques have been introduced to remove the image artifacts from the reconstructed images. A postprocessor can process a reconstructed image signal to reduce or remove the image artifacts resulting from quantization distortions. The general rule of thumb is that image edges should be preserved while rest of the image should be smoothed with spatial filters carefully chosen based on the characteristic of a particular pixel or set of surrounding pixels.
Most of the current post processing techniques have focused on reducing the artifacts produced by DCT coding, in particular, reducing the blocking artifacts. These techniques operate along block boundaries, and have difficulty in removing the ringing artifact that extends over a larger area. Some examples of these types of techniques can be found in U.S. Pat. No. 5,850,294, issued Dec. 15, 1998; U.S. Pat. No. 5,512,956, issued Apr. 30, 1996; and U.S. Pat. No. 5,473,384, issued Dec. 5, 1995.
The '294 patent, for example, uses one-dimensional lowpass filters along the block boundaries to reduce visual artifacts, including blocking and ringing, in DCT coded images. The '956 patent performs three-dimensional post processing to reduce artifacts produced by block-based motion-compensated coding. Spatial and temporal filtering is applied and then a one-dimensional median filter is used in the spatial domain. The '384 patent provides a description of a post-filtering method with a temporal filter and a spatial filter. A multilevel median filter is applied in the spatial domain.
The techniques of the '384 patent can also remove certain amount of ringing noises. Most current techniques in this area, including the approach in the '384 patent have high computational complexity that makes them inefficient for many applications. Other techniques of this type can be found in U.S. Pat. No. 5,920,356, issued Jul. 6, 1999; and U.S. Pat. No. 5,819,035, issued Oct. 6, 1998. The '035 patent applies anisotropic diffusion on coded images to reduce ringing artifacts. Lowpass filtering is done by an iterative diffusion process. The '356 patent applies three-by-three lowpass filters in the spatial domain, and uses both temporal and edge characteristics of the video image to enhance the displayed image.
As mentioned above, these techniques have computation complexity and makes them inefficient for many applications. Also, most of the above techniques are directed specifically to DCT-based coding. Therefore, a more general purpose, efficient and simple technique for removing ringing artifacts from images reconstructed from any compression scheme would be useful.